Conventional measurement of trace elements in gas samples has presented a number of difficulties. For example, to detect the amount of water present in a sample of the helium coolant from a nuclear reactor, typically the helium sample is passed over a mirror which is cooled so that any water present in the gas condenses. Light is passed through the sample and directed at the mirror; as the water condenses a change in reflection indicates the amount of water present in the gas. However, the response time required to sense the water, particularly at very low concentrations, is relatively long. Additionally, the system is complex and requires considerable maintenance. Refrigeration is required, the mirror periodically becomes pitted and water must be removed from the mirror between tests making for low response time and repetition rate.
Infrared detectors may be employed to monitor humidity and other trace elements. In such systems radiation including a first wavelength band which is significantly absorbed by the trace element and a second wavelength band which is not significantly absorbed, is passed through a gas sample. The output intensities of the respective bands are compared to provide a measure of the concentration of the trace element in the sample. Problems arise, however, as the filters or other means for providing the individual wavelength bands of radiation age or become contaminated. This tends to cause variations in the measured values of the compared intensities that are not due to changes in concentration. To avoid erroneous concentration measurements frequent calibration is therefore required.
Further disadvantages are exhibited by optical cells which hold the sample gas for testing. For example, to accurately and continuously monitor the sample gas a smooth uninterrupted gas flow is desired in the area of the introduced beam of radiation. However, present techniques for introducing the sample gas into the optical cell tend to generate considerable turbulence and undesirable recirculation within the cell which inhibit a smooth uninterrupted flow and permit dirt and other contaminants to collect on the mirrors of the optical cell. This interferes with proper circulation of the sample gas and causes inaccurate concentration measurements.